This invention relates to a novel chromate-free, self-healing conversion coating that provides significant corrosion resistance to magnesium alloys, along with strong adhesion with an overlaying paint layer (primer). The coating liquid is a waterborne formulation, that when applied to magnesium alloy panels by immersion, leads to less corrosion than other chromate and non-chromate industry standards. Based on results for treated-only samples, this new coating offers corrosion protection similar to a state-of-the-art chromate-free anodized coating.
Many methods have been used to protect magnesium alloys by inhibiting corrosion or slowing down the reaction mechanisms, including use of conversion coatings, anodizing, electroplating and addition of corrosion inhibitors. Conversion coatings are among the more cost-effective, and therefore, widely used methods to provide a barrier between the metal and its surrounding environment. It also serves as a tie-layer to improve adhesion between the metal substrate and subsequent paint (primer and topcoat) layers. The most popular and reliable conversion coating for magnesium is a conversion coating containing hexavalent chromium (Cr6+), also known as hex-chrome or chromate. However, the use of Cr6+ has been drastically curtailed in recent years as it has been found to be carcinogenic. Due to environmental concerns, recent efforts are concentrated on making chromate-free conversion coatings.
Different groups have used Vanadium-containing compounds, in one form or another, as an alternative to hexavalent chromium. Some of the relevant patents are: U.S. Pat. No. 4,828,615 is directed to the use of pentavalent vanadium, subsequent to a conversion coating. US Patent application No. 2003/0150526 A1 relates to a conversion coating comprising a source of vanadate ions, a material comprising phosphorus, source of nitrate ions, preferably with borate ions and fluoride ions. Patent application US 2004/0216637 A1 and U.S. Pat. No. 7,135,075, discuss an inorganic corrosion resistant coating with self-healing properties comprising a vanadate salt as a film forming agent, a supplemental soluble metal anion and a substrate activator. U.S. Pat. No. 6,887,320 mentions a process for applying a chromate-free, corrosion resistant coating comprising the steps of degreasing, cleaning, deoxidizing and immersing in a solution containing phosphate and fluoride ions with sodium tungstate and sodium vanadate as an active corrosion inhibitor. US Patent Applications 2008/0254315 A1 and US 2011/0041958, relate to an acidic chromium free solution for treating a metal surface comprising a vanadium cation and/or a vanadyl cation, an anion from an organic acid and an anion selected from the group consisting of oxoacids of nitrogen, sulfur, phosphorus, boron and chlorine. U.S. Pat. No. 7,964,030 B1 describes a vanadate solution for conversion treating a magnesium alloy containing metavanadate ion, and a polyhydroxylated aromatic compound in water.
As mentioned above, the use of vanadium (V4+ or V5+) in the presence of nitrate ions in a certain concentration range has been claimed. However, these coating formulations also require either, an activator or a corrosion inhibitor and/or supplemental ions to further promote film formation. For example, US Patent application No. 2003/0150526 A1 relates to a conversion coating comprising a source of vanadate ions, a material comprising phosphorus, source of nitrate ions, preferably with borate ions and fluoride ions. Phosphorus-based treatment results in a hard layer of insoluble phosphate, which is contiguous and highly adherent to the underlying metal. In contrast to all the above described coating inventions, the present invention does not require any supplement ions for the formation and stability of the conversion coating.
The chromate-free, self-healing conversion coating of the present invention provides an order of magnitude better corrosion resistance compared to previously reported conversion coatings. The superior protection of the conversion coating is attributed to the creation of a unique structure and morphology that serves as a barrier coating that simultaneously has self-healing properties. A hydroxoaluminate complex forms the backbone of the barrier coating, while the magnesium hydroxide domains facilitate the “slow release” of vanadium moieties to the defect sites, thus providing active corrosion protection. This synergistic performance using environmentally friendly chemicals leads to the enhanced corrosion protection.